MOSgated devices are well known and may have a planar channel geometry or a trench channel geometry.
In the planar geometry version, spaced channel regions are diffused into the surface of a chip and MOSgates cover the invertible channel regions which are coplanar with one another. Such structures are useful over a wide range of breakdown voltages.
In the trench geometry version, invertible channel regions are formed along the vertical walls of U-shaped trenches etched into the silicon surface. A source contact is connected to the channel region and source region for each separate trench unit. Trench devices are preferably used for lower breakdown voltage ratings, for example, less than about 100 volts.
Both planar geometry devices and trench geometry devices may be formed with channel regions of a spaced polygonal or spaced stripe arrangement.
Trench geometry devices have an inherently lower capacitance between gate and source, and thus a lower charge Qg than planar devices. Since an important figure of merit of a MOSFET is the product of Qg and the on-resistance RDSON, trench devices are frequently desired for low voltage applications requiring a minimum switching loss such as the MOSFETs used in low voltage power supplies for supplying power from a battery to a portable electronic device such as a lap top computer.
Trench device geometries have not permitted the best trench density for minimizing the RDSON. Therefore, while the trench device has a low Qg, complex manufacturing processes are needed to produce a low RDSON as well.
Thus, it is desirable to provide a trench geometry MOSgated device such as a MOSFET, which has a minimized Qg and RDSON but is capable of inexpensive and reliable production techniques.